In general paper making processes comprise a step where pulp enters a pressurised head box where it is agitated in water for optimum dispersion in the water. The pulp dispersion is then dispensed through a slit in the head box onto a moving wire screen where it forms a continuous web. The web is partly de-watered by gravity drainage, and is subject to suction which removes additional water. The web structure is then consolidated on a series of roll presses where additional water is removed. The web is further dried on steam-heated cylinders where the remaining water is evaporated to produce a dry paper sheet. One machine known in the art to perform this process is a Foudrinier-type paper mill.
In the above paper making process, air may be entrapped in the water used in this process, for example, during pulp agitation in the head box and during drainage where free-draining water, also called white water, falls into collecting trays. These collecting trays funnel the white water into a wire pit which is generally placed under a moving wire screen. To release the air in the white water within the wire pit, it is generally desirable to run the white water through a relatively long section of open channel. The resultant water is then recirculated in the paper making process, i.e., generally to dilute the pulp mixture in the headbox.
The white water in the above process may also contain substantial quantities of materials such as surfactants, fatty soaps, cellulose derivatives, inks and process chemicals such as sizing aids. Many of these materials cause foaming of the white water when it is agitated in presence of air. This foaming becomes more of a problem as the white water is reused in the process since the concentration of such materials increases at each recirculation. Often the temperature of the white water also increases steadily and white water may be at a temperature of from 50 to 70.degree. C.
Air may be entrapped in the water used in the paper making process in several ways. It is important to control this air entrapment so that the dry paper sheet has a uniform quality. Specifically, if air entrapment in the white water is not restricted, the strength of the wet web may be lowered and the paper sheet may have increased porosity, reduced smoothness and decreased strength.
Several methods of restricting air entrapment in white water of the paper making process have been proposed in the art. For instance, it has been proposed to use mechanical means to restrict the amount of air entrapped or to allow the air to escape after it has been entrapped. Similarly, it has been proposed to use chemical foam control agents such as C.sub.7 -C.sub.22 alcohols, polyalkylene glycols, fatty acids, fatty acid esters, amides, and organic phosphates. These approaches, however, are still in need of improvements.
Similarly, polysiloxane based foam control agents for aqueous systems have been used in the paper making process. The use of these materials may be limited, however, because they may cause spotting in the dry paper sheet. Moreover, polysiloxanes may interfere with the physical properties of the paper and they may interfere with the printing and coating process of paper.
U.S. Pat. No. 3,528,929 describes a foam control composition which comprises finely divided silica dispersed into a mixture of mineral oil, a hydrophobic agent and an alkoxysilicon chloride. This composition is taught to assist in de-aeration and drainage during the paper making process. Silica may, however, have detrimental effects on the paper and/or the paper mill.
It is, therefore, desired to provide a method which restricts air entrapment in the white water in the paper making process and which obviates the problems of the prior art processes. We have surprisingly found that certain siloxane based materials are especially useful as foam control agents in the paper making process, despite the above mentioned concerns about polysiloxanes.